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When I was growing up, we learnt a catchy phrase to help us remember the order of planets in the solar system. And it went a little something like this: “My very easy method just speeds up naming planets”. Or to give them their full names, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Of course these days, you’d have to lose the word ‘planet’, since Pluto no longer qualifies as one. And why I knew this fact, I didn’t know why. So of course I turned to the arbiter of all things space – NASA.

And here’s what they had to say on the topic..

Why is Pluto not classified as a planet anymore?

In 2003, an astronomer saw a new object beyond Pluto. The astronomer thought he had found a new planet. The object he saw was larger than Pluto. He named the object Eris (EER-is).

Finding Eris caused other astronomers to talk about what makes a planet a “planet.” There is a group of astronomers that names objects in space. This group decided that Pluto was not really a planet because of its size and location in space. So Pluto and objects like it are now called dwarf planets.

Pluto is also called a plutoid. A plutoid is a dwarf planet that is farther out in space than the planet Neptune. The three known plutoids are Pluto, Eris and Makemake (MAH-kee-MAH-kee). Astronomers use telescopes to discover new objects like plutoids.

Scientists are learning more about the universe and Earth’s place in it. What they learn may cause them to think about how objects like planets are grouped. Scientists group objects that are like each other to better understand them. Learning more about faraway objects in the solar system is helping astronomers learn more about what it means to be a planet.

So there you have it. Size does matter. Wonder if Earth will ever come in for a category change?